UAE said on Thursday it carried out air strikes in Yemen's Aden against "terrorist organisations" that attacked Saudi-led coalition forces and the city's airport, according to a statement quoted by UAE state news agency WAM.
UAE called for an international response against the attackers.
Yemen's government earlier on Thursday accused the UAE of launching air strikes against its troops in the interim capital Aden in support of separatists who say they have regained control of the southern city.
"The Yemeni government condemns the Emirati air strikes against government forces in the interim capital Aden and in Zinjibar, which resulted in civilian and military casualties," Deputy Foreign Minister Mohammad al-Hadhrami said in a tweet.
``We hold the UAE fully responsible for this explicit extra-judicial targeting'' of the government forces, the statement said, adding that the airstrikes also left several civilians dead but without providing a specific death toll.
He urged Saudi Arabia "to stand by the legitimate government and stop this illegal and unjustified military escalation".
Also Yemeni President Abd-Rabbu Mansour Hadi calls on Saudi Arabia to intervene to stop UAE interference and support for separatists and to halt air strikes against Yemeni govt forces, SABA news agency reported.
The Saudi-led coalition intervened in the war in 2015 to back up the internationaly recognised government after the Houthis swept south from their northern stronghold to seize the capital Sanaa and much of Yemen, the Arab world's poorest nation.
"The Coalition remains committed to supporting reforms in the legitimate government, tackling corruption, encouraging inclusiveness towards to all Yemeni factions, in order to ensure representation of all Yemenis in the future of the country," said Jaber Al Lamki, executive director of media and strategic communications at the National Media Council.
The government said on Wednesday it had seized back Aden from separatists who captured the strategic port city on August 10 after a fierce battle that left at least 40 people dead.
Last week, the UAE denied the government's claims it was "fully responsible for the armed rebellion".
Yemeni Information Minister Muammar al-Iryani tweeted on Thursday that the alleged UAE strikes had killed 40 combatants and wounded 70 civilians.
The fighting has opened a new front in a complex war that has already claimed tens of thousands of lives, sparking widespread malnutrition and what the United Nations calls the world's worst humanitarian crisis.
"It's total chaos here. There was fighting in the city all day yesterday. Things appear to have calmed down a bit this morning, but we expect the hostilities to resume at any point," MSF programme manager Caroline Seguin said in a statement.
"The situation is very fragile. Families are again trapped in their homes by fighting, unable to secure food and reach medical care," said the UN humanitarian coordinator in Yemen, Lise Grande.
The NGO Save the Children said its staff in Aden described a climate of terror.
"The situation is very tense; people are scared of what will happen next," acting country director Jason Lee said in a statement.
The UN said at least 13 people were reported killed and 70 wounded during clashes this week.
Separatists Regain Control
Col. Mohamed al-Oban, a commander of the government's special forces in Abyan province, said the troops were on the road, headed from Abyan toward Aden on Thursday, when the strikes took place, killing at least 30.
The deputy foreign minister did not say when the alleged air strikes took place.
But residents in Aden told AFP they heard air raids late Wednesday when forces loyal to Yemeni President Abedrabbo Mansour Hadi entered Aden, which separatists had captured earlier this month.
On Thursday a spokesman for the separatists' Southern Transitional Council (STC) told AFP the Security Belt Forces were again in full control of Aden.
A government security source confirmed Aden was under the full control of the STC, saying government troops who had entered the city a day earlier had withdrawn to Abyan province.
Sources on both sides said Thursday that clashes had flared in provincial capital Zinjibar, about 100 kilometres (62 miles) from Aden, after government troops withdrew to the area.
The clashes between separatists and government forces -- who for years fought on the same side against the Houthis -- have raised fears that the country could break apart entirely.
The United Nations Security Council on Thursday voiced concern over the clashes, including what it called a "violent attempt to take over state institutions".
In a statement, it urged all parties to "show restraint and to preserve Yemen's territorial integrity".
The separatists have brought massive reinforcements from other regions, officials from both sides said.
STC spokesman Haitham Nezar said the Security Belt forces were now setting their sights on the nearby provinces of Abyan and Shabwa, which had been retaken by government troops earlier this week.
"Our plan is to kick out the invading forces from the south," said Nezar, referring to government troops the separatists see as outsiders.
The Yemeni government has also drafted in reinforcements from the north as the two parties appear to be preparing for a major showdown.
STC vice president Hani bin Breik said that STC forces fighting the Houthis in the north had been sent to the south for a major battle.
"We will not remain in the fronts to liberate the north from the Houthis while the north is invading us," he said.
The STC is fighting to regain the independence of South Yemen, which unified with the north in 1990.
* This story was edited by Ahram Online.